Charge rolls installed in an image forming apparatus such as an electrophotographic copying machine or printer, are held in contact with a photoconductive (PC) drum for charging the circumferential surface of the PC drum. These charge rolls are used in a so-called “roll charging” method which is one of the known methods for charging a PC drum on which an electrostatic latent image is formed. In the roll charging method, the charge roll to which a charging voltage is applied is held in contact with the outer circumferential surface of the PC drum. The charge roll evenly charges the PC drum before the surface is locally exposed to a laser beam.
In general, charge rolls can include an electrically conductive center shaft (metal core) and an electrically conductive elastic layer which has a low hardness. In some instances, the elastic layer consists of either a foamed body or a solid rubber body whose hardness may be reduced by adding softener. The electrically conductive elastic layer can be formed on the outer circumferential surface of the center shaft with a suitable thickness. Some form of conductive primer or adhesive may be applied to the outer circumferential surface of the shaft before forming the elastic layer. A resistance adjusting layer and a protective layer can be applied to the outer circumferential surface of the conductive elastic layer.
A charge roll with an elastic layer consisting of a solid rubber body can have both a resistance adjusting layer and a protective layer consisting of a polyamide or polyurethane or the like that is spray or dip coated onto the surface of the elastic layer.
In other charge rolls, a foamed rubber can be employed for the elastic layer. When foamed rubber is used some form of adhesive or primer can be applied to the outer circumferential surface of the elastic layer. The resistance adjusting layer and protective layer can be formed as a multilayered sleeve or tube. This sleeve is adhered with the primer or adhesive or shrunk with a process such as heating over the elastic layer to immobilize the sleeve on the elastic layer. Multilayer sleeves or tubes can be produced by extrusion of the base layer and spray or dip coating to produce the outer layer. These process can be costly and can introduce significant variability in the charge roll surface due to the coating process.
Charge rolls constructed with an adhesive or primer on the outer circumferential surface of the elastic layer can also pose problems in controlling the bulk resistivity of the assembled charge roll. In the construction described, the adhesive or primer typically contains conductive additives such as carbon black, ionic salts, or the like. In addition, the method of applying the adhesive or primer in a continuous layer can be costly and is rarely efficient. The presence of adhesive or primer on the sleeve also renders it non-recyclable for future use on a new shaft and elastic layer.
Furthermore, charge rolls constructed with a solid rubber elastic layer have inherently higher hardness and thus can be less compliant. More compliance creates a wider nip, which compensates for problems such as PC drum runout, resulting in better charge roll to PC drum contact. More compliance also reduces the noise generated from the frequency of the AC/DC charge applied to a hollow, metal PC drum. Solid rubber charge rolls can also leach extractables that can contaminate the PC drum surface. The extractables include, but are not limited to, oils, softeners, conductive additives, curatives, and other materials which can be removed from the polymer matrix with solvents which include but are not limited to, methanol, acetone, and chloroform. Solid rubber rolls can be subject to non-uniformities in surface resistance due to thickness variation in the outer layers of the roll which are typically spray or dip coated.